In general, in a liquid-phase oxidation reaction, a dissolution of a catalyst component often becomes a problem. This is a phenomenon in which the catalyst component is dissolved out, and which may be considered a problem inherent to the liquid-phase oxidation reaction. For solving such problem, there is adopted a method of combining a base metal with a precious metal constituting the principal catalyst component and causing such base metal to be dissolved out, as in a palladium-copper catalyst often employed in a Wacker type oxidation reaction (Reference 1 or 2), palladium-heteropoly acid employed in a synthesis of acetic acid by an ethylene process (Reference 3), or palladium-tellurium employed in a synthesis of glyceric acid (Reference 4), namely sacrificing the base metal thereby suppressing the dissolution of the precious metal.
The dissolution of the catalyst component, being triggered by an oxidation of the catalyst component, may be suppressed by reducing a concentration or a partial pressure of oxygen, but such method will retard the oxidation reaction which is an intended main reaction. Consequently the method of sacrificing the base metal has been often employed as an unavoidable method, and a solution for such situation has been desired.    [Reference 1] JP 5-163179 A    [Reference 2] JP 5-294859 A    [Reference 3] JP 2002-540942 A    [Reference 4] JP 5-245373 A